Is there an error with Low Kick on the damage calculator, or am I doing something wrong? By comparing the Smogon Damage Calc with the Pikalytics damage calc, it looks like Low Kick's damage is receiving a hidden +1 boost (slightly more, maybe due to rounding):
Smogon damage calc for level 50 mons, doubles, no field effects other than sun, no boosts applied to any stats:
252 Atk Life Orb Technician Marshadow Low Kick (120 BP) vs. 252 HP / 4 Def Groudon: 146-173 (70.5 - 83.5%) -- guaranteed 2HKO
Possible damage amounts: (146, 148, 149, 152, 153, 156, 156, 157, 160, 161, 164, 165, 168, 169, 172, 173)
252 Atk Life Orb Marshadow Spectral Thief vs. 252 HP / 4 Def Groudon: 74-87 (35.7 - 42%) -- guaranteed 3HKO
Possible damage amounts: (74, 74, 75, 75, 78, 78, 78, 79, 79, 82, 82, 83, 83, 86, 86, 87)
Pikalytics damage calc for same sets and conditions:
252 Atk Life Orb Marshadow Low Kick (120 BP) vs. 252 HP / 4 Def Groudon: 99-117 (47.8 - 56.5%) -- 80.9% chance to 2HKO
(99, 99, 101, 101, 103, 105, 105, 107, 107, 109, 110, 110, 113, 113, 114, 117)
252 Atk Life Orb Marshadow Spectral Thief vs. 252 HP / 4 Def Groudon: 74-87 (35.7 - 42%) -- guaranteed 3HKO
(74, 74, 75, 75, 78, 78, 78, 79, 79, 82, 82, 83, 83, 86, 86, 87)
Note that the amounts for Spectral Thief match, but Low Kick does not.
Pikalytics damage calc for same sets but giving Marshadow a +1 Attack boost:
+1 252 Atk Life Orb Marshadow Low Kick (120 BP) vs. 252 HP / 4 Def Groudon: 144-172 (69.5 - 83%) -- guaranteed 2HKO
(144, 146, 148, 149, 152, 153, 156, 156, 157, 160, 161, 164, 165, 168, 169, 172)
+1 252 Atk Life Orb Marshadow Spectral Thief vs. 252 HP / 4 Def Groudon: 109-130 (52.6 - 62.8%) -- guaranteed 2HKO
(109, 110, 113, 113, 114, 117, 117, 118, 121, 121, 122, 125, 125, 126, 129, 130)
The Low Kick calcs are now closer to the Smogon Damage Calc (although they differ by 1 damage point).
As much as I'd love for Marshadow to be able to chunk ~75% of Groudon's health with low kick, in an in-game test it only did a little bit less than 50%, more consistent with the Pikalytics calc than Smogon's. (The test was with lvl 100 mons and Groudon maybe had 0 def, but this wouldn't account for the difference).